Will the BICEP2 Results Hold Up?

Two months ago, astronomers announced that they had detected gravitational waves from the dawn of time, the strongest evidence yet that the universe experienced a titanic growth spurt in the first moments after the Big Bang, a phenomenon called cosmic inflation. Scientists and the media—including this blog—buzzed with the news. However, a controversy that began with online rumors now might call these findings into question.

The BICEP2 telescope at twilight. Credit: Steffen Richter, Harvard University

The signal the astronomers detected was a subtle one. Using a telescope at the South Pole called BICEP2, they picked up swirls in the polarization, or alignment, of the cosmic microwave background (CMB) radiation, the afterglow of the Big Bang. This pattern, they argued, resembled what one would expect from gravitational waves produced by inflation. These findings not only seemed to be the first direct evidence for inflation, but also were an image of the earliest gravitational waves, the ripples in spacetime predicted by Einstein’s theory of general relativity.

“Seeing evidence for such a phase of rapid expansion would be extremely exciting, and it is possible that that’s what BICEP2 is seeing,” said Raphael Flauger, a theoretical physicist at New York University and the Institute for Advanced Study in Princeton, New Jersey, who did not take part in BICEP2’s research. However, a controversy that began with rumors reported on a scientist’s blog now hints the swirls the BICEP2 group thought it saw in the CMB may have actually emerged from within the Milky Way.

The kerfuffle began on May 12 with a post from particle physicist Adam Falkowski at CERN on his blog Résonaances. To uncover the particular polarization pattern they detected, the BICEP2 group had to account for the “foreground” of polarized microwaves generated relatively nearby within the Milky Way, which could generate similar patterns. It’s like hearing a low rumble: before you can be convinced that it’s distant thunder, you have to make sure that it isn’t just trucks clattering down your street.

“The two most important foreground components are synchrotron emission from charged particles that are deflected by the magnetic field of the galaxy, and emission from dust,” Flauger said.

To account for the synchrotron radiation, scientists used data from the WMAP satellite. The controversy hinges on their handling of the dust emission, which relied on a preliminary map based on about 15 months of data from the European Space Agency’s Planck spacecraft. Falkowski noted the BICEP2 group might have misinterpreted the Planck data, thinking that it only contained emissions from the Milky Way when it also included unpolarized emissions from other galaxies. If the BICEP2 team did not account for this fact, they might have underestimated how polarized the foreground from the Milky Way actually was. This could mean the inflation signal the group thought it saw might only be a spurious result from Milky Way emissions.

The BICEP2 team stands by their results, and strongly denied the rumor that it admitted to any mistake. However, in a seminar given on May 15, Flauger suggested the BICEP2 team’s analysis of dust emissions may not be reliable.

The area of the sky the BICEP2 team analyzed is not very dusty; in fact, they intentionally chose a relatively clear patch of sky to reduce the confusion with dust. However, in doing so, they may have introduced additional uncertainty into their results. “In regions that don’t contain much dust, it is hard to measure how polarized it is,” Flauger said. This means the uncertainty regarding their interpretation “seems to be relatively large,” he explained.

An end to this controversy may come in October, when the Planck team intends to release its complete map of the polarization of the CMB. “It will help us understand the foregrounds from our galaxy better, and will hopefully tell us if the signal is evidence for inflation or just dust,” Flauger said. “Planck may even be able to do more than that and see hints of the inflationary signal directly.”

Other research projects able to spot ancient gravitational waves could also help clear up the controversy, such as the POLARBEAR experiment in Chile, the South Pole Telescope, or the Keck Array in Antarctica, which is led by the scientists behind BICEP2, said cosmologist Julian Borrill at Lawrence Berkeley National Laboratory and the University of California at Berkeley.

“It is possible that the signal of the gravitational waves is there, and it will survive in a more complete analysis,” Falkowski said. “Everyone will be immensely happy if it does.”

As to where the rumors came from and whether they are true, “I think we’re already past the rumors stage and we have entered into an experts’ discussion,” Falkowski said. “I don’t think that discussing the origin and the content of the rumors adds anything important at this point.”

Charles Choi

Charles Q. Choi has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.

This week, NASA announced that it will partner with the European Space Agency to send a 4,760-pound spacecraft into space to peer out over billions of galaxies in an effort to map and measure the universe. Its purpose: to investigate the mysteries of dark matter and dark energy.

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